Chapter 5
Control Structures II
Chapter Objectives
• Learn about repetition (looping) control structures
• Explore how to construct and use countcontrolled, sentinel-controlled, flag controlled, and
EOF-controlled repetition structures
• Examine break and continue statements
• Discover how to form and use nested control
structures
Why Is Repetition Needed?
• There are many situations in which the
same statements need to be executed several
times
• Example
– Formulas used to find average grades for
students in a class
The while Looping (Repetition)
Structure
• Syntax
while(expression)
statement
• Expression is always true
in an infinite loop
• Statements must change
value of expression to
false
Counter-Controlled while Loop
• Used when exact number of data or entry pieces is
known
• Syntax:
int N = //value input by user or specified in program;
int counter = 0;
while(counter < N){
statement(s);
counter++;
}
Sentinel-Controlled while Loop
• Used when exact number of entry pieces is
unknown but last entry (special/sentinel value) is
known)
• Syntax:
input first data item into variable;
while(variable != sentinel){
statement(s);
input a data item into variable;
}
Flag-Controlled while Loop
• Boolean value used to control loop
• Syntax:
boolean found = false;
while(!found){
statement(s);
if(expression)
found = true;
}
EOF(End of File)-Controlled
while Loop
• Used when input is from files
• Sentinel value is not always appropriate
• Syntax:
input the first data item;
while(not_end_of_input_file){
statement(s);
input a data item;
}
• read method with value –1 can be used as expression in
while
Programming Example:
Checking Account Balance
• Input file: customer’s account number, account balance at
beginning of month, transaction type (withdrawal, deposit,
interest), transaction amount
• Output: account number, beginning balance, ending
balance, total interest paid, total amount deposited, number
of deposits, total amount withdrawn, number of
withdrawals
Programming Example: Checking
Account Balance
• Solution:
–
–
–
–
Read data
EOF-controlled loop
switch structure of transaction types
Determine action (add to balance or subtract
from balance depending on transaction type)
Programming Example:
Fibonacci Number
•
•
Fibonacci formula for any Fibonacci sequence:
an = an-1 + an-2
Input: first two Fibonacci numbers in sequence,
position in sequence of desired Fibonacci
number (n)
–
–
–
•
int previous1 = Fibonacci number 1
int previous2 = Fibonacci number 2
int nthFibonacci = position of nth Fibonacci number
Output: nth Fibonacci number
Programming Example:
Fibonacci Number (Solution)
• code for while loop based on inputs:
int counter = 3; // Fibonacci 1 and 2 already known
while(counter <= nthFibonacci){
current = previous2 + previous1;
previous1 = previous2;
previous2 = current;
counter++;
}
• Final result found in last value of current
The for Looping (Repetition)
Structure
• Specialized form of while loop
• Simplifies the writing of count-controlled loops
• Syntax
for(initial statement; loop condition; update
statement)
{
statement(s);
}
The for Looping (Repetition)
Structure
• Execution:
– initial statement
executes
– loop condition
evaluated
– If loop condition
evaluates to true,
execute for loop
statement and execute
update statement
– Repeat until loop
condition is false
The for Looping (Repetition)
Structure
• Does not execute if initial condition is false
• Update expression changes value of loop control
variable, eventually making it false
• If loop condition is always true, result is an infinite
loop
• Infinite loop can be specified by omitting all three
control statements
• If loop condition is omitted, it is assumed to be true
• for statement ending in semicolon is empty; does not
effect program
Programming Example: Classify
Numbers
• Input: N integers (positive, negative, and
zeros)
int N = 20; //N easily modified
• Output: number of 0s, number of even
integers, number of odd integers
Programming Example: Classify
Numbers (Solution)
• Use a for loop to get and evaluate 20 numbers
from the user and input into switch structure
• switch(number % 2)
• case 0:
– if(number == 0) increment number of zeros;
– if(number != 0) increment number of even integers;
• case 1:
– increment number of odd integers
The do…while Loop (Repetition)
Structure
• Syntax
do{
statement(s);
}
while(expression);
• Statements executed first, then expression
evaluated
• Statement(s) executed at least once then
continued if expression is true
do…while Loop (Post-test Loop)
break Statements
• Used to exit early from a loop
• Used to skip remainder of switch structure
• Can be placed within if statement of a loop
– If condition is met, loop exited immediately
continue Statements
• Used in while, for, and do...while structures
• When executed in a loop, the remaining
statements in the loop are skipped; proceeds with
the next iteration of the loop
• When executed in a while/do…while structure,
expression evaluated immediately after continue
statement
• In a for structure, the update statement is executed
after the continue statement; then the loop
condition executes
Nested Control Structures
• Provides new power, subtlety, and
complexity
• if, if…else, and switch structures can be
placed within while loops
• for loops can be found within other for
loops
Nested Control Structures
(Example)
• for(int i = 1; i <= 5; i++){
for(int j = 1; j <= i; j++)
System.out.print(“*”);
System.out.println();
}
• Output:
*
**
***
****
*****
Chapter Summary
• Looping Mechanisms
–
–
–
–
–
–
Counter-controlled while loop
Sentinel-controlled while loop
Flag-controlled while loop
EOF-controlled while loop
for loop
do…while loop
• break statements
• continue statements
• Nested control structures